This invention relates to a printed circuit board assembly.
A standard has developed whereby many electronic products are mounted in racks composed of vertical posts that are at a uniform horizontal spacing of around 48 cm. The electronic product is installed in a casing or frame that is slightly narrower than the horizontal space between the vertical posts, and the frame is inserted into the rack and secured by attachment ears to the two posts. Further, a standard has developed such that the frames are of a uniform height of about 13.2 cm.
Many electronic products are sufficiently small that they do not occupy an entire frame of standard height and width. In such a case the interior space of the frame may be divided into compartments and multiple products, which may or may not be functionally similar, are installed in the compartments respectively. This modular product approach not only avoids waste of space in the frame but also allows the user to select for installation in a given frame the particular modules that meet his requirements.
In one implementation of the modular product concept, each module comprises two main components, namely a backplane assembly that is attached to the frame at the rear, and a main circuit board assembly that is inserted into the appropriate compartment from the front of the frame. The backplane assembly comprises a connector board provided with connectors for connection to signal input and output (I/O) cables and the main circuit board assembly comprises a front panel which constitutes the user interface and is provided with, for example, indicator lamps and controls, and a main circuit board which extends into the compartment from the front panel. The leading edge of the main circuit board assembly carries one part of an edge connector, a complementary part of which is attached to the backplane assembly and is engaged when the main circuit board assembly is properly installed in its compartment. It may be necessary for the edge connector that connects the backplane assembly to the main circuit board assembly to have 32 or more pins.
The modular product concept has found acceptance in video signal processing applications.
Video signal processing is a connection-intensive field, and it might be desirable for the backplane assembly of a single module whose connector board is about 3 cm wide to carry twelve or more I/O connectors. These connectors may be BNC connectors. A BNC connector is a coaxial connector that is substantially cylindrical and about 0.95 cm in diameter, and generally occupies an area of the connector board that is about 1 cm square. The centers of BNC connectors must be spaced by at least 1.6 cm in order to avoid difficulty in connecting and disconnecting cables from the connectors. If the BNC connectors are arranged in two rows and the centers of the rows are 1.6 cm apart, the horizontal width of the strip-form region between the areas occupied by the two rows is only about 0.5 cm. This implies that any connector that is mounted on the connector circuit board between the rows of BNC connectors must be narrower than 0.5 cm.
One type of connector that is used to interconnect circuit boards is known as the DIN connector. The male portion, or header, of a DIN connector comprises a housing of insulating material formed with three rows of holes. The holes in a row are about 0.25 cm apart, and the rows are about 0.17 cm apart. Metal pins are fitted in the holes. The housing of the connector is about 1 cm wide. In one version, the housing is about 9.2 cm long and is provided with 96 pins in three rows of 32 pins each. Although this type of connector has more than enough pins for connecting to the main circuit board of many modular products, it cannot be accommodated on the connector circuit board because the housing is too wide to fit between the two rows of BNC connectors spaced at less than about 2 cm.
Another type of connector that is in use is known as the single in-line, or SIP, connector. The header of a SIP connector is about 0.3 cm wide and the space between pins is the same as the DIN connector. The SIP connector is not designed for frequent connections and disconnections.
An attempt has been made to accommodate a standard DIN connector in the backplane assembly of a modular product by including an interface board in the backplane assembly. The interface board is attached to the connector board by a row of attachment pins that project from the connector board at right angles between the two rows of BNC connectors and pass through respective holes in the interface board. The pins are bent over through 90 degrees so that the interface board extends perpendicular to the connector board. The attachment pins also provide electrical connection between circuit traces of the connector board and circuit traces of the interface board. The interface board is provided with one portion of the DIN connector along its edge farther from the connector board. The circuit traces of the interface board provide connections between the attachment pins and the pins of the DIN connector. A power supply is fitted in one compartment of the frame, and power is supplied to the interface board through a ribbon cable having a receptacle that is releasably engaged with a header on the interface board.
This attempt to solve the problem of connecting the main circuit board assembly to the backplane assembly is subject to the disadvantage that the conductor traces of the interface board are quite long, resulting in a high potential for crosstalk between the circuit traces. Further, the attachment pins are soldered to plated through-holes in both the connector board and the interface board and therefore the interface board cannot readily be detached from the connector board. Use of the attachment pins for both mechanical and electrical connection is unsatisfactory, since mechanical stresses may impair the electrical connection. The use of attachment pins does not lead to a rugged assembly, particularly since the pins lie in a single row and the major linear dimension of the interface board is perpendicular to that row. This design does not lend itself well to wider modules, having four rows of BNC connectors, since the traces in the connector board connecting the more distant rows to the attachment pins would have to be rather long.